The present invention relates to a method for separating plutonium ions from aqueous solutions containing sulfuric acid, in which plutonium is initially transferred, by liquid-liquid extraction with an organo-phosphoric acid ester in an organic solvent, from an aqueous phase to an organic phase, and then re-extracted with an aqueous salt solution.
Plutonium containing sulfate solutions are obtained during the treatment of plutonium-containing combustible wastes with sulfuric acid, in, for example, the so-called acid digestion process. Plutonium-containing sulfuric acid solutions may also be encountered in the processing or reprocessing of nuclear fuels. The separation and isolation of plutonium from such solutions is an important task within the framework of nuclear technology because plutonium constitutes a valuable nuclear fuel and because plutonium may also contribute to a significant increase in the risk potential in connection with the final storage of wastes. The ability to quantitatively separate plutonium is therefore desirable. The known, industrially utilized processes for separation of plutonium, such as extraction from nitric acid solutions in the Purex process, cannot be used with sulfuric acid solutions since the behavior of plutonium in the extraction process is different in the presence of sulfate ions.
There are references in the literature which discuss reagents that have been used in the direct extractive separation of plutonium from sulfate solutions. Use of such reagents is discussed in D. E. Horner, C. F. Coleman, USAEC Report, ORNL-2830 (1959); D. E. Horner, C. F. Coleman, USAEC Report, ORNL-3051 (1961); and A. Zolotov, M. K. Shmutova, P. N. Palei, J. Anal Chem. USSR 21, 1079 (1966). Examples of the reagents which have been discussed in the literature are primary amines, such as Primene JM-T, a commercially available homologue mixture of primary amines with highly branched alkyl groups having an average of about 20 carbon atoms, made by Rohm & Haas, 1-phenyl-3-methyl-4-benzoyl-pyrazolone-5 (PMBP), trioctylphosphine oxide (TOPO) and di-2-ethylhexylphosphoric acid (D2EHPA). In the reported experiments, the plutonium content of the solutions to be extracted was between 2 and 20 mg Pu per liter. In all of the reported experiments, the oxidation state (IV) of plutonium was stabilized by the addition of 0.05 to 0.5 M sodium nitrite. The sequence of the ease of extractability for the various valencies of the plutonium was the same for all of the extraction agents that were examined, and was Pu.sup.IV &gt;Pu.sup.VI .gtoreq.Pu.sup.III &gt;Pu.sup.V.
The primary amine Primene JM-T, dissolved in Amsco 125-82, a commercially available hydrocarbon diluent mixture, was examined most intensively, with a view towards the possibiities for industrial utilization. With this amine, extraction coefficients of 8000 or more were realized with H.sub.2 SO.sub.4 concentrations from 0.5 to 2.5 M. The extraction coefficients decreased with increasing acid concentration, and were highest with the presence of 2 M (NH.sub.4).sub.2 SO.sub.4 in 0.5 M H.sub.2 SO.sub.4. To prevent the formation of a third phase, the extraction agent was modified with a long chain alcohol.
By re-extraction of the plutonium from the organic phase with 2 to 5 M HNO.sub.3, a product solution was obtained which contained, depending on the amine concentration and on the extracted metal sulfate complexes, considerable quantities of sulfate ions. However, as noted above, the presence of sulfate ions makes the product solution not suitable for further processing in the Purex process. Reductive re-extraction is impossible. Moreover, the extraction agent, which now contains nitric acid, must be purified, since nitric acid interferes with the extraction of plutonium.
In the Zolotov et al reference, a 0.1 M PMBP solution in benzene was used to exract Pu(IV) from solutions containing 0.5 to 3.5 M H.sub.2 SO.sub.4 with extraction coefficients (D) equal to 35 to 85. An increase in the acid concentration resulted in a poorer extraction.
The re-extraction took place with 7.5 M H.sub.2 SO.sub.4 or with a 10 M HCl solution. In neither case, however, could the resulting solution be used in the Purex process. It has also been found possible to re-extract the plutonium by twice contacting the organic phase with a saturated oxalic acid solution.
When TOPO dissolved in Amsco 125-82 was used as the extraction agent, the extraction coefficients for Pu(IV) in 3 to 5 M H.sub.2 SO.sub.4 were between 20 and 30. The addition of sodium sulfate caused a worsening of the extraction of Pu(IV), and the addition of sodium nitrate, to a final concentration of 1 to 2 M, had no influence on the extraction coefficient.
In the re-extraction from 0.3 M TOPO with a 1 M Na.sub.2 CO.sub.3 solution, re-extraction coefficients of more than 1000 were realized. However, the re-extraction solution contained sulfate ions, since 1 mole of sulfate ions per 2 moles of TOPO reached the organic phase.
The lowest extraction coefficients for Pu(IV) were realized with D2EHPA dissolved in Amsco 125-82, as the extraction agent. For an extraction from a solution containing 3 to 5 M H.sub.2 SO.sub.4, the coefficient was between 4 and 10. In the presence of 0.5 M Na.sub.2 SO.sub.4 in 3 M H.sub.2 SO.sub.4, the extraction coefficient increased to 50.
Re-extraction of Pu(IV) with 8 to 10 M HNO.sub.3 or ferrous sulfamate was possible only if the D2EHPA concentration was less than 0.01 M. If a 0.01 M D2EHPA solution was contacted with 0.1 to 1.0 M oxalic acid solution, re-extraction coefficients of 100 to 2000 resulted. Another effective re-extraction medium was a 1 M Na.sub.2 CO.sub.3 solution. Depending on the plutonium content of the organic phase (20 mg to 1.1 g Pu), the resulting re-extraction coefficients were between 20 and 80.
In order to avoid the formation of a third phase due to the presence of the sodium salt of the dialkyl phosphoric acid, it was necessary, in an alkali re-extraction, to modify the organic phase by the addition of long chain alcohols, alkyl phosphates, alkyl phosphonates or alkyl phosphine oxides. With the addition of tributyl phosphate (TBP) the extraction worsened, and TOPO had only a slight effect on the extraction coefficient. A synergistic effect was noted with a mixture of 0.1 M TOPO and 0.4 M D2EHPA only for the extraction of Pu(IV). Thus, the addition of 0.1 M TOPO to 0.4 M D2EHPA increased the extraction coefficient from 0.4 to 5.
In view of the desired further processing of the product solution as a nitric acid solution in the Purex process it was impossible, in the prior art, to use Primene JM-T. The same applied to TOPO for the extraction of gram quantities of plutonium. Moreover, PMBP is a special reagent and, in view of its limited availability on the market and its present high cost, it is of no economic interest. The use of PMBP, TOPO and D2EHPA according to the prior art methods, however, require multi-stage separating processes in order to realize sufficiently high separation factors.